JP2012142101A - Lithium sulfur battery and method for manufacturing lithium sulfur battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium sulfur battery capable of further improving charge/discharge efficiency, and a manufacturing method thereof.SOLUTION: A coin type battery 20 comprises a cup-shaped battery case 21, a negative electrode 22 provided inside the battery case 21, a positive electrode 23 provided at a position opposed to the negative electrode 22 via a separator 24, a nonaqueous electrolyte 27 containing a supporting salt, a gasket 25 formed of an insulation material, and a hole sealing plate 26 provided at an opening of the battery case 21 and sealing the battery case 21 via the gasket 25. Here, the negative electrode 22 has a negative electrode active material containing a lithium base material, on which a lithium carboxylate film is formed. The positive electrode 23 has a positive electrode active material containing sulfur.

Description

  The present invention relates to a lithium-sulfur battery and a method for producing a lithium-sulfur battery.

  Conventionally, sulfur batteries using sulfur as a positive electrode active material are known. Sulfur has an extremely high theoretical capacity density of 1672 mAh / g and is expected as a high capacity battery. However, in electrolyte-based sulfur batteries, sulfur molecules, polysulfide ions, etc. dissolve and diffuse into the electrolyte during charge / discharge, etc., and this reacts with the anode metal to cause self-discharge or deteriorate the anode. As a result, the battery capacity and charge / discharge efficiency (Coulomb efficiency) may be deteriorated.

  Thus, for example, a cathode containing an electroactive sulfur material, an anode containing lithium, a non-aqueous electrolyte, and one or more selected from the group consisting of acyclic ethers, cyclic ethers, polyethers, and sulfones An electrochemical cell having a non-aqueous solvent, one or more lithium salts, and one containing one or more N—O additives has been proposed (see Patent Document 1). This electrochemical cell is said to exhibit any of a low self-discharge rate, a high cathode utilization rate, a high charge-discharge efficiency, and a high specific capacity.

Special table 2007-518229 gazette

  However, in the thing of patent document 1, when the polysulfide ion diffused in the non-aqueous electrolyte is oxidized by the N—O additive, the amount of sulfur that can contribute to the battery reaction as an active material decreases. There was a thing. Thereby, charging / discharging efficiency may fall and it was desired to improve charging / discharging efficiency more.

  This invention is made | formed in order to solve such a subject, and it aims at providing the manufacturing method of the lithium sulfur battery which can improve charge / discharge efficiency more, and a lithium sulfur battery.

  In order to achieve the above-mentioned object, the present inventors have found that when a lithium-sulfur battery is produced using a lithium-based material on which a lithium carboxylate film is formed as a negative electrode material, the charge / discharge efficiency can be further increased. Has led to the completion of the present invention

That is, the lithium sulfur battery of the present invention is
A positive electrode having a positive electrode active material containing sulfur;
A negative electrode having a negative electrode active material including a lithium-based material on which a lithium carboxylate film is formed;
An ion conductive medium interposed between the positive electrode and the negative electrode;
It is equipped with.

In addition, the method for producing a lithium-sulfur battery of the present invention includes
(A) a film forming step of immersing a lithium-based material in a treatment liquid containing a carboxylic acid to form a lithium carboxylate film on the lithium-based material;
(B) a positive electrode having a positive electrode active material containing sulfur, a negative electrode having a negative electrode active material containing a lithium-based material on which the lithium carboxylate film is formed, and an ion conducting medium interposed between the positive electrode and the negative electrode; A battery constituting step of constituting a battery comprising:
Is included.

  In this lithium-sulfur battery, charge / discharge efficiency can be further increased. The reason why such an effect is obtained is not clear, but it is presumed that self-discharge at the negative electrode and deterioration of the negative electrode are suppressed when the lithium carboxylate film is formed on the negative electrode surface.

Sectional drawing showing the outline of a structure of the coin-type battery 20 Explanatory drawing of the evaluation cell 30. FIG. The IR spectrum of the negative electrode of Experimental Examples 1-3. The graph which shows the relationship between the acetic acid density | concentration of immersion liquid, discharge capacity, and charging / discharging efficiency. The graph which shows the relationship between charging / discharging frequency | count, charging / discharging capacity | capacitance, and charging / discharging efficiency.

  Next, an embodiment embodying the present invention will be described. The lithium-sulfur battery of the present invention includes a positive electrode having a positive electrode active material containing sulfur, a negative electrode having a negative electrode active material containing a lithium-based material on which a lithium carboxylate film is formed, and ions interposed between the positive electrode and the negative electrode A conductive medium.

In the lithium-sulfur battery of the present invention, the positive electrode is, for example, a mixture of a positive electrode active material, a conductive material, and a binder, and an appropriate solvent is added to form a paste-like positive electrode material on the surface of the current collector. It may be dried and compressed to increase the electrode density as necessary. The positive electrode active material contains sulfur. Sulfur may be contained in any form, but is preferably either or both of elemental sulfur and metal sulfide. In the present invention, the metal sulfide includes a metal polysulfide. That is, when the metal is Me, the metal sulfide can be expressed by Me _x S _y (x ≧ 0, y ≧ 0).

  In the lithium-sulfur battery of the present invention, the negative electrode has a negative electrode active material. This negative electrode active material includes a lithium-based material on which a lithium carboxylate film is formed. For this negative electrode, for example, a negative electrode active material, a conductive material, and a binder are mixed, and an appropriate solvent is added to form a paste-like negative electrode material, which is applied to the surface of the current collector and dried. The electrode may be compressed to increase the electrode density, but it is not necessary to use a conductive material, a binder, or a current collector.

  Here, the lithium-based material refers to a material containing lithium, and includes lithium oxide, lithium composite oxide, lithium sulfide, lithium composite sulfide, and the like in addition to lithium metal and lithium alloy. Examples of the lithium alloy include alloys of lithium with aluminum, silicon, tin, magnesium, indium, calcium, and the like. This lithium-based material is preferably a material mainly composed of lithium. Here, the main component can be a component having the largest atomic weight ratio among all the components contained, for example, a component contained at 50 at% or more, a component contained at 70 at% or more, 90 at% or more It is good also considering the component contained etc. as a main component. The lithium-based material is preferably metallic lithium. This is because it is considered that the use of metallic lithium as the negative electrode can increase the theoretical voltage and the electrochemical equivalent and can increase the capacity. The lithium-based material is preferably a material that can occlude and release lithium.

  The lithium carboxylate film formed on the lithium-based material is preferably formed on the surface of the lithium-based material, and preferably formed so as to cover the surface of the lithium-based material. It does not have to be covered. For example, the surface of the lithium material may be formed in a net shape or an island shape.

The lithium carboxylate is preferably a lithium salt of at least one of a monocarboxylic acid and a dicarboxylic acid. Especially, it is preferable that it is 1 or more types of RCOOLi and _Rx (COOLi) ₂ , _Rx (COOH) (COOLi), and it is more preferable that it is at least one of RCOOLi and (COOLi) ₂ . Here, x is 0 or 1. In addition, when x = 0, it becomes lithium oxalate in which carbons of (COOLi) are bonded to each other. R is a hydrogen group or a hydrocarbon group. Here, the hydrocarbon group is preferably a chain-like (which may be linear or branched) or a cyclic hydrocarbon group, and preferably has 1 to 20 carbon atoms. Examples of chain hydrocarbon groups include methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decanyl, undecanyl, and dodecanyl groups. An unsaturated hydrocarbon group such as a vinyl group, a propenyl group, a butenyl group, a pentenyl group, a heptenyl group, an octenyl group, a nonenyl group, a decenyl group, an undecenyl group, and a dodecenyl group; Examples of the cyclic hydrocarbon group include cyclic saturated hydrocarbon groups such as cyclopropyl group, cyclopentyl group, cyclohexyl group, and cycloheptyl group; aromatic hydrocarbon groups such as phenyl group and naphthyl group. These hydrocarbon groups may have a substituent or a hetero atom in the structure. Examples of the substituent include a halogen, an alkoxy group, a nitro group, and a cyano group. The hydrocarbon group may have a plurality of substituents of the same type or may have a plurality of different types of substituents. Of these, the lithium carboxylate is preferably at least one of lithium formate (HCOOLi), lithium acetate (CH ₃ COOLi), lithium oxalate (COOLi) ₂ , and lithium trifluoroacetate (CF ₃ COOLi). .

The lithium carboxylate is preferably formed by immersing the above-described lithium-based material in a treatment liquid containing carboxylic acid. This treatment liquid may be obtained by dissolving carboxylic acid in a solvent. The carboxylic acid contained in the treatment liquid is not particularly limited as long as it can form the above-described lithium carboxylate, and is preferably at least one of a monocarboxylic acid and a dicarboxylic acid. Among these, at least one of RCOOH and R _x (COOH) ₂ is preferable. Here, x is 0 or 1. R may be the same as the hydrogen group or hydrocarbon group described above. As such a carboxylic acid, at least one of formic acid (HCOOH), acetic acid (CH ₃ COOH), oxalic acid (COOH) ₂ , and lithium trifluoroacetate (CF ₃ COOH) is preferable. The details of the treatment liquid and the conditions for immersing the lithium-based material in the treatment liquid will be described in detail in the description of the method for manufacturing a lithium-sulfur battery described later, and thus description thereof is omitted here. In addition, you may use what added carboxylic acid to the ion conduction medium (electrolytic solution) of a lithium sulfur battery as a processing liquid.

  The lithium carboxylate may be formed as a coating on a lithium-based material in a battery by adding the above-described carboxylic acid as an additive to the electrolyte and bringing the electrolyte into contact with the lithium-based material (hereinafter referred to as a film). Then, it is also referred to as a mode of addition). Further, the lithium carboxylate may be formed in advance as a film on the surface of the lithium-based material by immersing the lithium-based material in the treatment liquid before constituting the battery (hereinafter also referred to as an immersion mode). Thus, charging / discharging efficiency can be improved by forming a film in the aspect added and the aspect immersed. Among these, the aspect to immerse is more preferable. If the immersion mode is adopted, it is not necessary to add an additive necessary for forming a lithium carboxylate film in the battery system, and inhibition of the battery reaction due to the additive can be suppressed, so that the capacity can be further increased. Because it is considered. For example, it is considered that the capacity can be further increased because the additive used for forming the coating can suppress a decrease in the amount of the sulfur component due to the reaction with the sulfur component that can contribute to the battery reaction as an active material. .

  In the lithium-sulfur battery of the present invention, the conductive material used for the positive electrode and the negative electrode is not particularly limited as long as it is an electron conductive material that does not adversely affect the battery performance. For example, natural graphite (scale-like graphite, scale-like graphite) Graphite such as artificial graphite, acetylene black, carbon black, ketjen black, carbon whisker, needle coke, carbon fiber, metal (copper, nickel, aluminum, silver, gold, etc.), etc. Can be used. Among these, carbon black, ketjen black, and acetylene black are preferable as the conductive material from the viewpoints of electron conductivity and coatability. The binder serves to bind the active material particles and the conductive material particles. For example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), fluorine-containing resin such as fluorine rubber, or polypropylene, Thermoplastic resins such as polyethylene, ethylene-propylene-dienemer (EPDM), sulfonated EPDM, natural butyl rubber (NBR) and the like can be used alone or as a mixture of two or more. In addition, an aqueous dispersion of cellulose or styrene butadiene rubber (SBR), which is an aqueous binder, can also be used. Examples of the solvent for dispersing the active material, the conductive material, and the binder include ethanol, N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, and N, N-dimethylamino. Organic solvents such as propylamine, ethylene oxide, and tetrahydrofuran can be used. Moreover, a dispersing agent, a thickener, etc. may be added to water, and an active material may be slurried with latex, such as SBR. As the thickener, for example, polysaccharides such as carboxymethyl cellulose and methyl cellulose can be used alone or as a mixture of two or more. Examples of the application method include roller coating such as applicator roll, screen coating, doctor blade method, spin coating, bar coater, and the like, and any of these can be used to obtain an arbitrary thickness and shape. Current collectors include aluminum, titanium, stainless steel, nickel, iron, calcined carbon, conductive polymer, conductive glass, and aluminum, copper, etc. for the purpose of improving adhesion, conductivity, and oxidation resistance. A surface treated with carbon, nickel, titanium, silver or the like can be used. For these, the surface can be oxidized. Examples of the shape of the current collector include foil, film, sheet, net, punched or expanded, lath, porous, foam, and formed fiber group. The thickness of the current collector is, for example, 1 to 500 μm.

In the lithium-sulfur battery of the present invention, the ion conductive medium may be a solution in which a supporting salt is dissolved in a solvent. The supporting salt is not particularly limited as long as it is a lithium salt used in a normal lithium secondary battery. For example, lithium bis (trifluoromethanesulfonyl) imide (LiTFSI), Li (C ₂ F ₅ SO ₂ ) Known supporting salts such as ₂ N, LiPF ₆ , LiClO ₄ , and LiBF ₄ can be used. These may be used alone or in combination. The solvent is not particularly limited, but is preferably a non-aqueous solvent, carbonates such as ethylene carbonate (EC), diethyl carbonate (DEC) and propylene carbonate (PC), dimethoxyethane (DME), triglyme and Ethers such as tetraglyme, cyclic ethers such as dioxolane (DOL) and tetrahydrofuran, and mixtures thereof are preferred. Alternatively, ionic liquids such as 1-methyl-3-propylimidazolium bis (trifluorosulfonyl) imide and 1-ethyl-3-butylimidazolium tetrafluoroborate can be used. The ion conductive medium may be gelled by adding an electrolyte containing a supporting salt to a polymer such as polyvinylidene fluoride, polyethylene oxide, polyethylene glycol, polyacrylonitrile, or a saccharide such as an amino acid derivative or a sorbitol derivative. .

  The lithium sulfur battery of the present invention may include a separator between the negative electrode and the positive electrode. The separator is not particularly limited as long as it can withstand the range of use of the electricity storage device. For example, a polymer nonwoven fabric such as a polypropylene nonwoven fabric or a polyphenylene sulfide nonwoven fabric, a microporous film of an olefin resin such as polyethylene or polypropylene, And polyimide three-dimensional porous film. These may be used alone or in combination.

  The shape of the lithium-sulfur battery of the present invention is not particularly limited, and examples thereof include a coin type, a button type, a sheet type, a laminated type, a cylindrical type, a flat type, and a square type. Moreover, you may apply to the large sized thing etc. which are used for an electric vehicle etc.

  Next, the manufacturing method of a lithium sulfur battery is demonstrated. This manufacturing method includes (a) a film forming step for forming a lithium carboxylate film on a lithium-based material used for the negative electrode, and (b) a battery constituting step for forming a battery.

(A) Film formation process In this process, a lithium-based material is immersed in a treatment liquid containing a carboxylic acid to form a lithium carboxylate film on the lithium-based material. As the lithium-based material, those shown in the description of the lithium-sulfur battery described above can be applied.

The treatment liquid containing carboxylic acid may be a solution obtained by dissolving carboxylic acid in a solvent. The carboxylic acid contained in the treatment liquid is not particularly limited as long as it can form the above-described lithium carboxylate, and is preferably at least one of a monocarboxylic acid and a dicarboxylic acid. Among these, at least one of RCOOH and R _x (COOH) ₂ is preferable. As the treatment liquid containing such a carboxylic acid, those shown in the description of the lithium-sulfur battery described above can be applied.

  The solvent contained in the treatment liquid is not particularly limited, but is preferably a non-aqueous solvent, carbonates such as ethylene carbonate (EC), diethyl carbonate (DEC), and propylene carbonate (PC), dimethoxyethane (DME), Preferred are ethers such as triglyme and tetraglyme, cyclic ethers such as dioxolane (DOL) and tetrahydrofuran, and mixtures thereof. Alternatively, ionic liquids such as 1-methyl-3-propylimidazolium bis (trifluorosulfonyl) imide and 1-ethyl-3-butylimidazolium tetrafluoroborate can be used. Of these, ethers and cyclic ethers are preferred. Note that as such a solvent, a solvent of an ion conductive medium used for a lithium-sulfur battery may be used. In this case, the supporting salt contained in the ion conductive medium may be included, but those not containing the supporting salt are preferable. It is because it is thought that it is hard to produce a by-product.

  The concentration of the carboxylic acid in the treatment liquid is not particularly limited, but is preferably greater than 0M and 5.0M or less. Among these, 0.1M or more and 4M or less are preferable, 0.2M or more and 3M or less are more preferable, and 0.5M or more and 2.0M or less are more preferable. If it is such a range, a capacity | capacitance can be raised more and charging / discharging efficiency can be raised more. Further, the concentration of the carboxylic acid may be defined as described above, but the concentration of the (COOH) structure may be defined with one (COOH) structure as 1 mol. In this case, the concentration of the (COOH) structure is preferably greater than 0M and 5.0M or less, more preferably 0.1M or more and 4.0M or less, and 0.2M or more and 3.0M or less. Further preferred. Note that M is synonymous with mol / L.

  The conditions for immersing the lithium-based material in the treatment liquid can be determined empirically depending on the type and concentration of the carboxylic acid in the treatment liquid. For this reason, although it does not specifically limit, For example, 1 minute or more is preferable, 10 minutes or more are more preferable, and 1 hour or more are further more preferable. It is because it is thought that a film will be formed if it is 1 minute or more. The immersion time is preferably 120 hours or less, more preferably 48 hours or less, and even more preferably 24 hours or less. This is because if the treatment is performed for 120 hours, the amount of the subsequent coating film formed is considered to be very small. Although the temperature at the time of immersion should just be below the temperature which volatilizes more than the temperature which a process liquid freezes, it is good also as 5 to 80 degreeC, for example, it is good also as 10 to 60 degreeC, and 20 degreeC. It is good also as 30 degreeC or less above.

(B) Battery configuration process In this process, a battery including a positive electrode, a negative electrode, and an ion conductive medium is configured. The shape of the battery to be configured is not particularly limited, and examples thereof include a coin type, a button type, a sheet type, a laminated type, a cylindrical type, a flat type, and a square type. Moreover, you may apply to the large sized thing etc. which are used for an electric vehicle etc. Here, a case where the coin-type battery shown in FIG. 1 is configured will be described as an example of a battery configuration process. FIG. 1 is a cross-sectional view schematically showing the configuration of the coin-type battery 20. In this battery configuration process, first, a cup-shaped battery case 21 is prepared, and a negative electrode 22 is disposed inside the battery case 21. The negative electrode 22 has a negative electrode active material containing a lithium-based material in which a lithium carboxylate film is formed in the above-described film forming process. Next, the separator 24 is placed over the negative electrode 22 while injecting the ion conductive medium 27 into the battery case 21. The ion conductive medium 27 to be injected is interposed between the positive electrode 23 and the negative electrode 22. Since this ion conduction medium is common with the ion conduction medium of the above-described lithium-sulfur battery, detailed description thereof is omitted here. Subsequently, the positive electrode 23 is disposed at a position facing the negative electrode 22 via the separator 24. Here, the positive electrode 23 has a positive electrode active material containing sulfur. Since this positive electrode is in common with the positive electrode of the above-described lithium-sulfur battery, detailed description is omitted here. Subsequently, a gasket 25 formed of an insulating material is provided, and an ion conductive medium 27 is additionally injected as necessary. Finally, the sealing plate 26 is disposed in the opening of the battery case 21, and the end of the battery case 21 is crimped to seal the battery case 21 to produce the coin-type battery 20. In addition, the above-mentioned method is an example of a battery structure process, and a battery structure process is not limited to what was illustrated here.

  In such a method for producing a lithium-sulfur battery of the present invention, a lithium carboxylate film can be formed on the negative electrode surface. For this reason, it is possible to suppress self-discharge at the negative electrode and deterioration of the negative electrode. In addition, since the film is formed without adding carboxylic acid to the ion conductive medium, it is possible to suppress the reduction of the activity of sulfur due to the reaction of carboxylic acid and sulfur contained in the ion conductive medium. . In this way, it is possible to provide a lithium-sulfur battery that can further increase capacity and charge / discharge efficiency. Note that the above-described lithium-sulfur battery of the present invention is not limited to those manufactured by this manufacturing method.

  It should be noted that the present invention is not limited to the above-described embodiment, and it goes without saying that the present invention can be implemented in various modes as long as it belongs to the technical scope of the present invention.

  Below, the example which produced the lithium sulfur battery of this invention concretely is demonstrated as an Example.

(1) Production of evaluation cell [Experimental example 1]
The positive electrode material was produced as follows. 50% by weight of sulfur powder (made by high-purity chemical, purity 99.99%) as an active material, 10% by weight of polytetrafluoroethylene as a binder, and carbon (ECP600JD, manufactured by Lion) as a conductive material 40% by weight was blended, kneaded in a mortar and then formed into a sheet. This sheet was cut into a circular shape having a diameter of 12 mm and dried by heating under vacuum to obtain a positive electrode material. The negative electrode material was produced as follows. A lithium foil cut into a circular shape having a thickness of 0.4 mm and a diameter of 18 mm is fixed to an evaluation cell 1 used in a charge / discharge test described later, and 24 in a 1,3-dioxolane (DOL) solution containing 1.0 M formic acid. It was immersed for a period of time to form a lithium formate film on the surface of the lithium foil. This immersion treatment was performed in a constant temperature phase maintained at 25 ° C. Hereinafter, the DOL solution containing carboxylic acid is also referred to as an immersion liquid. The thus obtained lithium foil having a lithium formate coating on its surface was used as the negative electrode material. The electrolyte was adjusted as follows. 1M lithium bis (trifluoromethanesulfonyl) imide (LiTFSI) was added to a DME / DOL solution in which 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) were mixed at a volume ratio of 9: 1. This was dissolved and used as an electrolytic solution. Using the negative electrode material, the positive electrode material, the electrolytic solution, and the polyethylene porous membrane as the separator thus obtained, an evaluation cell shown in FIG. 2 was prepared in a glove box under an argon atmosphere. did. Here, the evaluation cell used for the formation of the coating was used for the assembly described later, with the negative electrode material on which the coating was formed being discarded.

  2A and 2B are explanatory views of the evaluation cell 30, FIG. 2A is a cross-sectional view before the evaluation cell 30 is assembled, and FIG. 2B is a cross-sectional view after the evaluation cell 30 is assembled. In assembling the evaluation cell 30, first, a negative electrode 36 and a polyethylene separator 38 (a microporous polyethylene membrane, a microporous polyethylene film, Tonen Chemical Co., Ltd.) and the positive electrode 40 were laminated in this order while injecting 100 μl of the electrolyte into the cavity 34. Further, an insulating ring 49 made of polypropylene is inserted, and then a conductive column 44 fixed in a liquid-tight manner in the hole of the insulating ring 42 is disposed on the positive electrode 40, and a conductive cup-shaped lid 46 is formed in the cylinder. Screwed into the substrate 32. Further, an insulating resin ring 47 is arranged on the cylinder 44, and a pressure bolt 45 having a through hole 45a is screwed into a screw groove carved in an inner peripheral surface of an opening 46a provided in the center of the upper surface of the lid 46, The negative electrode 36, the separator 38, the solid electrolyte membrane 38, and the positive electrode 40 were pressed and adhered. Thus, the evaluation cell 30 was produced. Since the cylinder 44 is positioned below the upper surface of the ring 42 and is in contact with the lid 46 via the insulating resin ring 47, the lid 46 and the cylinder 44 are in an electrically non-contact state. Further, since the packing 48 is disposed around the cavity 44, the electrolyte injected into the cavity 34 does not leak to the outside. In this evaluation cell 30, the lid 46, the pressure bolt 45, and the cylindrical base body 32 are integrated with the negative electrode 36 so that the whole becomes the negative electrode side, and the column 44 is integrated with the positive electrode 40 and insulated from the negative electrode 36. Therefore, it becomes the positive electrode side. The evaluation cell thus obtained was designated as Experimental Example 1.

[Experimental Examples 2 to 4]
An evaluation cell was prepared through the same steps as in Experimental Example 1 except that an immersion liquid containing acetic acid was used instead of formic acid, and this was designated as Experimental Example 2. Further, an evaluation cell was produced through the same steps as in Experimental Example 1 except that an immersion liquid containing oxalic acid was used instead of formic acid, and this was designated as Experimental Example 2. In addition, an evaluation cell was produced through the same steps as in Experimental Example 1 except that an immersion liquid containing trifluoroacetic acid was used instead of formic acid.

[Experimental Examples 5 to 8]
An evaluation cell was prepared through the same steps as in Experimental Example 2 except that the acetic acid concentration of the immersion liquid was 0.2 M, and this was designated as Experimental Example 5. In addition, an evaluation cell was prepared through the same steps as in Experimental Example 2 except that the acetic acid concentration of the immersion liquid was 0.4 M, and this was designated as Experimental Example 6. An evaluation cell was prepared through the same steps as in Experimental Example 2 except that the concentration of acetic acid in the immersion liquid was 2.0 M, and this was designated as Experimental Example 7. An evaluation cell was prepared through the same steps as in Experimental Example 2 except that the concentration of acetic acid in the immersion liquid was 5.0 M, and this was designated as Experimental Example 8.

[Experimental Examples 9 to 12]
The same steps as in Experimental Example 1 were performed except that a film was not formed in advance on the surface of the lithium foil as the negative electrode material, and that 0.5 M formic acid was added as an additive during the preparation of the electrolytic solution. Then, an evaluation cell was produced, and this was designated as Experimental Example 9. An evaluation cell was prepared through the same steps as in Experimental Example 9 except that the electrolytic solution was adjusted using acetic acid instead of formic acid. Further, an evaluation cell was prepared through the same steps as in Experimental Example 9 except that the electrolyte was adjusted using oxalic acid instead of formic acid, and this was designated as Experimental Example 11. In addition, an evaluation cell was prepared through the same steps as in Experimental Example 9 except that the electrolytic solution was adjusted using trifluoroacetic acid instead of formic acid.

[Comparative Example 1]
An evaluation cell was produced through the same steps as in Experimental Example 1 except that no film was formed on the surface of the lithium foil as the negative electrode material, and this was designated as Comparative Example 1.

(2) IR measurement The IR spectrum was measured about the negative electrode material of Experimental Examples 1-3 which formed the film. The measuring apparatus used was an AVATAR360 Fourier transform infrared spectrometer manufactured by Nicorey. The wave number resolution was 1.929 cm ⁻¹ , the number of integrations was 32 times, and the reflection method was used as the measurement method. FIG. 3 is an IR spectrum of the negative electrode materials of Experimental Examples 1 to 3. Thus, lithium formate is formed on the surface of the negative electrode material of Experimental Example 1, lithium acetate is formed on the surface of the negative electrode material of Experimental Example 2, and lithium oxalate is formed on the surface of the negative electrode material of Experimental Example 3. I understood. For identification of the coating components, an organic compound spectrum database (SDBS) provided by the National Institute of Advanced Industrial Science and Technology was used.

(3) Evaluation of electrochemical characteristics Using the evaluation cells of Experimental Examples 1 to 12 and Comparative Example 1 obtained as described above, electrochemical characteristics were evaluated. In the evaluation of the electrochemical characteristics, the capacity per unit weight of the negative electrode material (mAh / g) and the charge / discharge efficiency (%) were examined. First, the evaluation cell was installed in a constant temperature layer maintained at 25 ° C., and a constant current discharge of 0.5 mA was performed in a region from 2.8 V to 1.5 V, and then a constant current charge was performed. This discharge and charge were repeated as 50 cycles for 50 cycles. And the discharge capacity and charge capacity at this time were measured. Hereinafter, the charge capacity at the nth cycle is C _n (mAh / g), and the discharge capacity at the nth cycle is D _n (mAh / g). Table 1 shows the discharge capacity D ₁ (mAh / g) and charge / discharge efficiency (%) = (D ₂ / C ₁ ) × 100 of Experimental Examples 1 to 8 and Experimental Examples 9 to 12.

(4) Consideration From Table 1, in Examples 1 to 7 in which a film was previously formed on the surface in a dipping mode, or in a mode to be added, a carboxylic acid was added as an additive to the electrolyte solution to form a film on the negative electrode surface. It was found that the charging / discharging efficiency of the experimental examples 9 to 12 is higher than that of the comparative example 1 that does not. The reason why such an effect was obtained was presumed that the shuttle effect could be suppressed by forming a film on the lithium surface. Moreover, it turned out that the thing of Experimental Examples 1-8 of the aspect to immerse has a higher discharge capacity than the thing of Experimental Examples 9-12 of the aspect to add. It was speculated that the reason why such an effect was obtained was that the polysulfide diffused in the electrolytic solution could be prevented from being consumed by reacting with the carboxylic acid contained as an additive. From the above, it was found that the immersion mode is preferable from the viewpoint of increasing the discharge capacity and increasing the charge / discharge efficiency. In addition, although the film was formed on the metal lithium surface here, it was guessed that the same effect will be acquired if it contains lithium.

  Further, from Table 1, the discharge capacity is highest in Experimental Example 2 containing acetic acid in the immersion liquid, followed by Experimental Example 3 containing oxalic acid, followed by Experimental Example 4 containing trifluoroacetic acid, and formic acid. It was found that Experimental Example 1 including From this, it was found that it is particularly preferable that the immersion liquid contains acetic acid from the viewpoint of increasing the discharge capacity. Further, it was found that acetic acid has a particularly high discharge capacity in the immersion mode than in the addition mode, and the significance of adopting the immersion mode is particularly high. From the viewpoint of charge / discharge efficiency, it has been found that since the experimental example 3 including oxalic acid in the immersion liquid has the highest charge / discharge efficiency, the immersion liquid preferably includes oxalic acid.

  FIG. 4 is a graph showing the relationship between the concentration of acetic acid in the solution, the discharge capacity, and the charge / discharge efficiency when the coating is formed. FIG. 4 also shows these approximate curves. From FIG. 4, when forming the lithium acetate film, the concentration of acetic acid in the solution is preferably more than 0M and 5.0M or less, more preferably 0.1M or more and 4.0M or less, It turned out that it is more preferable that they are 0.2M or more and 3.0M or less. Further, it was speculated that there was a suitable concentration range even when other than acetic acid such as formic acid, oxalic acid, and trifluoroacetic acid was used. Specifically, assuming that one (COOH) structure is 1 mol, the concentration of the (COOH) structure is preferably greater than 0M and 5.0M or less, more preferably 0.1M or more and 4.0M or less. It was estimated that 0.2M or more and 3.0M or less was more preferable.

  FIG. 5 is a graph showing the relationship between the charge / discharge frequency, charge / discharge capacity, and charge / discharge efficiency of Experimental Examples 1 to 3, 9 to 11, and Comparative Example 1. From this graph, it was found that the lithium carboxylate film formed according to the mode of addition or the mode of immersion was high in charge / discharge efficiency. Moreover, although the capacity | capacitance was low in the thing of the aspect added, it turned out that there is little deterioration of a capacity | capacitance. On the other hand, although the capacity | capacitance may deteriorate by repeating charging / discharging by the thing of the aspect to immerse, it turned out that a capacity | capacitance is high overall and an initial stage capacity | capacitance can be raised especially and it is preferable.

  20 coin-type battery, 21 battery case, 22 negative electrode, 23 positive electrode, 24 separator, 25 gasket, 26 sealing plate, 27 ion conduction medium, 30 evaluation cell, 32 cylindrical substrate, 34 cavity, 36 negative electrode, 38 separator, 40 positive electrode, 42 ring, 44 cylinder, 45 pressure bolt, 45a through hole, 46 lid, 46a opening, 47 resin ring for insulation, 48 packing, 49 insulation ring.

Claims (6)

A positive electrode having a positive electrode active material containing sulfur;
A negative electrode having a negative electrode active material including a lithium-based material on which a lithium carboxylate film is formed;
An ion conductive medium interposed between the positive electrode and the negative electrode;
Lithium sulfur battery equipped with.   The lithium sulfur battery according to claim 1, wherein the lithium carboxylate is one or more of lithium acetate, lithium oxalate, lithium trifluoroacetate, and lithium formate.   The lithium-based material on which the lithium carboxylate film is formed is obtained by immersing the lithium-based material in a treatment liquid containing a carboxylic acid and previously forming a lithium carboxylate film on the lithium-based material. Or the lithium sulfur battery of 2. (A) a film forming step of immersing a lithium-based material in a treatment liquid containing a carboxylic acid to form a lithium carboxylate film on the lithium-based material;
(B) a positive electrode having a positive electrode active material containing sulfur, a negative electrode having a negative electrode active material containing a lithium-based material on which the lithium carboxylate film is formed, and an ion conducting medium interposed between the positive electrode and the negative electrode; A battery constituting step of constituting a battery comprising:
A method for producing a lithium sulfur battery comprising:   The method for producing a lithium-sulfur battery according to claim 4, wherein the treatment liquid containing the carboxylic acid has a carboxylic acid concentration of 5.0 M or less.   The method for producing a lithium-sulfur battery according to claim 4 or 5, wherein the treatment liquid containing the carboxylic acid contains at least one of formic acid, acetic acid, oxalic acid, and trifluoroacetic acid as the carboxylic acid.

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